1. Field of the Invention
This invention pertains to crystal-controlled oscillators, especially oscillators included in a video circuit to generate signals having crystal-controlled frequencies that are useful in processing a video signal.
2. Description Relative to the Prior Art
Sometimes it is necessary to utilize several locally-generated video frequency signals in the processing of a signal bearing video information. For instance, a color subcarrier signal of 3.58 mHz is ordinarily generated in a video player and used to reconstruct a composite video signal in a form suitable for display by a color television receiver. In addition, one or more delay lines are frequently employed for drop-out compensation, comb filtering and like functions requiring video signal delay. With a clock-driven delay device, such as a charge-coupled device (CCD), a local source of a clock frequency signal is required.
A simple and direct way to obtain the necessary local frequencies is to dedicate a crystal-controlled oscillator to the generation of each needed frequency signal. In the case of the color subcarrier and the CCD clock signals, frequencies generated for these purposes will be incorporated in the spectrum of the output video signal. The color subcarrier signal is there as an important component of the reconstructed composite signal. But the clock signal also feeds through the delay line and into the video signal; subsequent filtering attentuates most . . . but not all . . . of the clock signal, leaving a remnant in the output video signal. This frequency remnant is the problem: Unless the frequency of the clock signal exactly tracks the corresponding harmonic frequency of the color subcarrier signal, the harmonic of the subcarrier signal will mix (heterodyne) with the attenuated clock signal and a spurious difference frequency will appear in the composite signal. With separate oscillators as frequency sources this problem is difficult to avoid since temperature changes, circuit aging and crystal differences virtually assure slight frequency differences, even if the two oscillators can be started in phase and at the same frequency. The spurious difference frequency will ordinarily be a relatively low frequency and appear as visible interference in a picture reproduced from the composite signal.
Phase locking is a technique for keeping two frequency signals in lockstep phase, and therefore at the same frequency. U.S. Pat. No. 3,996,610 describes a video processing system with two phase-locked loops, one loop for operating a clock-controlled delay line (used as a comb filter) and the other for providing a color subcarrier signal to reconstruct a composite video signal. In this patent the two phase-locked loops are essentially independent: the delay line's clock is frequency-locked to the frequency of the recovered color burst signal (which is varying due to fluctuations in the recovery apparatus) and the color subcarrier frequency is locked to a 3.58 mHz frequency from a crystal oscillator. Though the nominal clock frequency (10.7 mHz) is a multiple of the color subcarrier frequency (3.58 mHz), these frequencies are not locked together.
Although the phase-locked loop provides a technical approach to the problem of unwanted signal mixing, it has practical difficulties since it is a complex circuit involving several components. Besides a voltage-controlled oscillator, each phase-locked loop requires a phase detector. In many pieces of consumer equipment it is desirable to reduce the number of parts and simplify the circuits as much as possible, since complexity often equates with cost. Except for the problem of heterodyning, a crystal oscillator is often preferred.